Methods and apparatus for positioning a CT imaging x-ray beam

ABSTRACT

In one embodiment, the present invention is a method for positioning an x-ray beam on a multi-slice detector array of an imaging system in which the detector array has rows of detector elements and is configured to detect x-rays in slices along a z-axis. The method includes steps of comparing data signals representative of x-ray intensity received from different rows of detector elements and positioning the x-ray beam in accordance with a result of the comparison.

BACKGROUND OF THE INVENTION

This invention relates generally to computed tomography (CT) imagingand, more particularly, to methods and apparatus for positioning anx-ray beam in a multi-slice CT imaging system.

In at least one known computed tomography (CT) imaging systemconfiguration, an x-ray source projects a fan-shaped beam which iscollimated to lie within an X-Y plane of a Cartesian coordinate systemand generally referred to as the “imaging plane”. The x-ray beam passesthrough the object being imaged, such as a patient. The beam, afterbeing attenuated by the object, impinges upon an array of radiationdetectors. The intensity of the attenuated beam radiation received atthe detector array is dependent upon the attenuation of the x-ray beamby the object. Each detector element of the array produces a separateelectrical signal that is a measurement of the beam attenuation at thedetector location. The attenuation measurements from all the detectorsare acquired separately to produce a transmission profile.

In known third generation CT systems, the x-ray source and the detectorarray are rotated with a gantry within the imaging plane and around theobject to be imaged so that the angle at which the x-ray beam intersectsthe object constantly changes. A group of x-ray attenuationmeasurements, i.e., projection data, from the detector array at onegantry angle is referred to as a “view”. A “scan” of the objectcomprises a set of views made at different gantry angles, or viewangles, during one revolution of the x-ray source and detector. In anaxial scan, the projection data is processed to construct an image thatcorresponds to a two-dimensional slice taken through the object. Onemethod for reconstructing an image from a set of projection data isreferred to in the art as the filtered back projection technique. Thisprocess converts the attenuation measurements from a scan into integerscalled “CT numbers” or “Hounsfield units”, which are used to control thebrightness of a corresponding pixel on a cathode ray tube display.

In a multi-slice system, movement of an x-ray beam penumbra overdetector elements having dissimilar response functions can cause signalchanges resulting in image artifacts. Opening system collimation to keepdetector elements in the x-ray beam umbra can prevent artifacts butincreases patient dosage. Known CT imaging systems utilize a closed-loopz-axis tracking system to position the x-ray beam relative to a detectorarray. It would be desirable to provide a closed-loop system thatoperates during patient scanning to maintain the x-ray beam penumbra inthe z-axis at a position relative to a detector array edge to minimizepatient dosage, but far enough away from the edge to reduce artifacts.

BRIEF SUMMARY OF THE INVENTION

There is therefore provided, in one embodiment, a method for positioningan x-ray beam on a multi-slice detector array of an imaging system inwhich the detector array has rows of detector elements and is configuredto detect x-rays in slices along a z-axis. The method includes steps ofcomparing data signals representative of x-ray intensity received fromdifferent rows of detector elements and positioning the x-ray beam inaccordance with a result of the comparison.

The above described embodiment and systems performing this methodperiodically adjust the x-ray beam position to maintain the beampenumbra at a minimal distance from the detector array edge, so thatpatient dosage is minimized and imaging artifacts are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of a CT imaging system.

FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1.

FIG. 3 is a schematic view of a portion of the CT imaging system shownin FIG. 1 showing an embodiment of a z-axis position system of thepresent invention.

FIG. 4 is a flow diagram an embodiment of a z-axis tracking loop of thepresent invention.

FIG. 5 is a flow diagram of a method for calibrating tracking loopparameters.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system 10is shown as including a gantry 12 representative of a “third generation”CT scanner. Gantry 12 has an x-ray source 14 that projects a beam ofx-rays 16 toward a detector array 18 on the opposite side of gantry 12.Detector array 18 is formed by detector elements 20 that together sensethe projected x-rays that pass through an object 22, for example amedical patient. Each detector element 20 produces an electrical signalthat represents the intensity of an impinging x-ray beam and hence theattenuation of the beam as it passes through patient 22. During a scanto acquire x-ray projection data, gantry 12 and the components mountedthereon rotate about a center of rotation or isocenter 24.

Rotation of gantry 12 and the operation of x-ray source 14 are governedby a control mechanism 26 of CT system 10. Control mechanism 26 includesan x-ray controller 28 that provides power and timing signals to x-raysource 14 and a gantry motor controller 30 that controls the rotationalspeed and position of gantry 12. A data acquisition system (DAS) 32 incontrol mechanism 26 samples analog data from detector elements 20 andconverts the data to digital signals for subsequent processing. An imagereconstructor 34 receives sampled and digitized x-ray data from DAS 32and performs high-speed image reconstruction. The reconstructed image isapplied as an input to a computer 36 that stores the image in a massstorage device 38.

Computer 36 also receives commands and scanning parameters from anoperator via console 40 that has a keyboard. An associated cathode raytube display 42 allows the operator to observe the reconstructed imageand other data from computer 36. The operator supplied commands andparameters are used by computer 36 to provide control signals andinformation to DAS 32, x-ray controller 28 and gantry motor controller30. In addition, computer 36 operates a table motor controller 44 thatcontrols a motorized table 46 to position patient 22 in gantry 12.Particularly, table 46 moves portions of patient 22 through gantryopening 48.

In one embodiment, and as shown in FIG. 3, x-ray beam 16 emanates from afocal spot 50 of x-ray source 14 (FIG. 2). X-ray beam 16 is collimatedby collimator 52, and collimated beam 16 is projected toward detectorarray 18. Detector array 18 is fabricated in a multi-slice configurationand includes detector element rows 54, 56, 58 and 60 for projection datacollection. A plane 86, generally referred to as the “fan beam plane”,contains the centerline of focal spot 50 and the centerline of beam 16.Fan beam plane 86 is illustrated in FIG. 3 as being aligned with acenterline D₀ of detector array 18, although fan beam plane 86 will notalways be so aligned. Detector element rows 62, 64, 66 and 68 serve asz-position detectors for determining a z-axis position of x-ray beam 16.In one embodiment, detector rows 62, 64, 66, and 68 are rows of detectorarray 18. Outer rows 62 and 68 are selected to be at least substantiallywithin penumbra 70 of beam 16. Inner rows 64 and 66 are selected to beat least substantially within umbra 72 of beam 16. “At leastsubstantially within” means either entirely within or at leastsufficiently within so that outer row 62 and 68 signal intensitiesdepend on an x-ray beam position and inner row 64 and 66 signalintensities provide references against which outer row signals arecompared. In one embodiment, collimator 52 includes tapered cams 74 and76. (Where it is stated herein that a cam “has a taper,” it is notintended to exclude cams having a taper of zero unless otherwisestated.) X-ray controller 28 controls positioning of cams 74 and 76.Each cam can be independently positioned to alter position and width ofx-ray umbra 72 relative to an edge (not shown) of detector array 18.

As shown in FIG. 4, one embodiment of a closed-loop method forpositioning beam 16 comprises comparing signals representative of x-rayintensity received from different rows of detector elements andpositioning an x-ray beam in accordance with results of the comparison.In one embodiment, signals representative of x-ray intensity fromdetector rows 62, 64, 66 and 68 are summed 78 to obtain row sums. Thesummation is over views taken in a 20-millisecond interval. For example,after the analog signals are converted to digital format, hardwarecircuitry (not shown) in DAS 32 performs offset correction anddetermines row sums from signals received from outer row 62 and frominner row 64. A corrected ratio R is determined 80 by determining aratio of a sum of signals received from outer row 62 to a sum of signalsreceived from inner row 64 and multiplying the ratio by a ratiocorrection factor. The ratio correction factor, determined from imagingsystem 10 calibration, accounts for different relative DAS gains betweenouter row 62 and inner row 64.

Beam position Z(R) then is determined 82, in millimeters relative to acenterline. Beam position Z is obtained by applying a predetermined beamposition transfer function to the corrected ratio to calculate the x-raybeam position. The beam position transfer function Z(R) is represented,for example, by a fourth-degree polynomial having predeterminedcoefficients:

Z(R)=a+bR+cR²+dR³+eR⁴

Beam position transfer function Z(R) and its limits are specified atimaging system 10 calibration.

A new collimator position is then determined 84. A focal spot position fis determined 84 from beam position Z, current collimator position C andother system 10 geometric parameters in accordance with:$f = {\frac{\left( {Z - C - T_{z}} \right)}{{fm}_{zz}\left( l_{fs} \right)} + C + T_{z}}$

where T_(z) represents a current taper of cam 74, fm_(zz) represents afocal spot magnification factor at rows 62 and 64 and is a function offocal spot size, and l_(fs) represents focal spot 50 length. A newposition for collimator 52 then is determined 84 for a detector element20 positioned toward isocenter 24. Collimator 52 is repositioned wherean edge (not shown) of collimator 52 would meet a line between focalspot position f and a target beam position Z_(t) which has beenspecified at imaging system 10 calibration. New collimator positionC_(n) thus is determined in accordance with:$C_{n} = {\frac{\left( {Z_{t} - f} \right)}{{cm}_{i}\left( l_{fs} \right)} + f}$

where cm_(i) represents a current collimator magnification factor atdetector element 20 positioned toward isocenter 24 and is a function offocal spot size, and l_(fs) represents focal spot 50 length.

In one embodiment, steps 78, 80, 82, and 84 are performed independentlyfor each side of collimator 52 at intervals to continuously obtain newpositions for each side of collimator 52. These intervals are, in oneembodiment, 20 milliseconds, to sample the x-ray beam 16 position 25times during a 0.5 second scan to minimize control loop lag error.However, in other embodiments, the interval is between 5 millisecondsand 50 milliseconds. In still other embodiments, the interval is betweena minimum value sufficient to avoid effects of quantum noise and highfrequency variation (such as due to x-ray tube anode movement at a runfrequency between 50 Hz and 160 Hz) and a maximum contrained by a slewrate of the sag curve. Sampling the changing sag curve frequently avoidsexcessive positioning error. (Sag is a periodic movement of x-ray beam16 resulting from gravity and from centrifugal forces acting onmechanical structure during a rotation of gantry 12.)

During patient scanning, z-position detectors 62, 64, 66 or 68 maybecome blocked by patient clothing, blankets, or other object. Afterblockage of a z-position detector 62, 64, 66, or 68 has been detected,or when x-ray source 14 first turns on, the loop sample interval isadjusted downward. In one embodiment, the loop sample interval isadjusted downward to 5 milliseconds. After 4 milliseconds ofstabilization, the position of the beam is measured and collimatorpositioning is started to further minimize initial position errors.

During a blockage, loop operation is suspended. To determine if anyz-position detectors are blocked, a signal from a last data detectorelement 90 adjacent a z-position detector 62, 64, 66 or 68 is comparedto an expected signal Sx. Z-position detector blockage is assumed, inone embodiment, if a last data detector element 20 signal is less than0.9 times expected signal Sx. In other embodiments, detector blockage isassumed when a last data detector element 20 signal is less than a valuebetween 0.95 and 0.5 times expected signal Sx. (It is desirable to makethis value as large in magnitude as possible to identify patientblockage as quickly as possible, thereby avoiding mispositioning ofx-ray beam 16 due to corrupted Z-measurement data. A maximum of 0.95 isused in one embodiment because it is known that x-ray scatter blockagefrom large patients 22, for example, can reduce a signal to 0.95 timesthe expected value.) During a blockage, collimator positioning issuspended. However, position measurement continues at an interval thatis decreased from 20 to 5 milliseconds. The decreased measurementinterval allows imaging system 10 to more quickly detect an end of theblockage and to resume closed-loop positioning.

Expected signal Sx is written as:

Sx=gmA*csf*t*g,

where gmA is a generator current mA signal proportional to an x-raysource 14 energizing current, csf is a scale factor determined at system10 calibration, t is a DAS sample time period, and g is a gain factor.Gain factor g allows expected signal Sx to be adjusted according to again value used for scanning. In one embodiment, this gain value isselectable from a plurality of gain values available in system 10.

In one embodiment, closed loop tracking is suspended when signalcorruption is detected. Signal corruption is detected, for example, bydetermining an actual focal spot length from a beam position and acollimator position, and comparing the actual focal spot length to anominal focal spot length. When a difference of, for example, more than0.1 millimeter is detected between the actual focal spot length and thenominal spot length, corruption is assumed to exist and collimatorpositioning is suspended. (In other embodiments, a difference thresholdfor assuming corruption is as small as 0.05 millimeter or as large asabout 0.6 millimeter. In still other embodiments, a value is selectedbetween a lower limit set by higher probabilities of false activationdue to noise, x-ray scatter and/or momentary beam position disturbancesand an upper limit that still provides some of the advantages oftracking.) However, beam position measurement continues at a decreasedinterval, as when a blockage is detected. Such corruption may occur, forexample, for a short time just prior to or just following detection of apatient blockage. If the corruption persists, for example, over 90° ofrotation of gantry 12 without detecting a patient blockage, amalfunction of the tracking system requiring servicing has likelyoccurred. In such an event, a scan is immediately aborted to avoidpatient dose and collection of non-diagnostic quality images. In otherembodiments, a limit is set from as little as 45° to as much as 360° ofa rotation of gantry 12. In other embodiments, a limit is set between avalue at which a false alarm rate due to scatter and/or an occasionalexceptionally long partial patient 22 blockage is acceptable and anupper limit representing a design choice as to how long compromisedoperation (high dose and/or non-diagnostic quality images) can betolerated before terminating a scan.

After system 10 has been switched off, position of focal spot 50 changesas source 14 cools over time. In one embodiment, before system 10 isswitched on again, an initial focal spot position is approximated frominformation obtained when a focal spot position was last measured. Anapproximation of a linear function is used to model focal spot positionchange during cooling in one embodiment, and in another embodiment, thelinear function is a 97 nanometer per second linear function. Becauseposition change with cooling is an exponential function, the linearapproximation is clamped at 0.15 millimeters. This clamping correspondsto approximately 20% of a cooling change in system 10 when fully cold,where a linear approximation to the exponential function suffices. Afully cold position requires 8 to 12 hours without patient scanning, anda tube warm up prior to patient scanning is normally requested if thetube has been off more than 1 hour. Therefore, a fully cold position,although possible, is not likely during normal patient scanning. Duringtube warm up a current measured position of the focal spot isestablished again for initial positioning of the collimator.

Several tracking loop parameters described herein, specifically, beamposition transfer function Z(R) and its limits and target beam positionZ_(t), are determined at system 10 calibration. FIG. 5 illustrates oneembodiment of a method for calibrating tracking loop parameters. In thisembodiment, data from a stationary sweep scan is collected 100 whilecollimator 52 is stepped through a sequence of z-axis positions. Beam 16is incremented 0.3 millimeters on detector array 18 exposure surface foreach collimator 52 step position. The sweep scan data isoffset-corrected and view averaged 102 to obtain a set of detectorsamples for each collimator 52 step position. A position of the focalspot is then determined 104. A collimator 52 z-axis position offset fromdetector array centerline D₀ is determined 104, as the point where outerrows 62 and 68 receive signals of half-maximum intensity at fulldetector element 20 width. Position of focal spot 50 during sweep scanthen is determined 104 from collimator 52 z-axis offset and nominalsystem 10 geometric parameters.

A beam 16 position is determined 106 for each detector element 20 ateach collimator 52 step position. Beam 16 positions are determined fromsweep scan focal spot 50 position, nominal length of focal spot 50, andnominal system 10 geometry.

Target beam position Z_(t) then is determined 108 for detector element20 positioned toward isocenter 24. When beam 16 is directed at targetbeam position Z_(t), beam 16 is sufficiently close to detector array 18edge 92 to prevent imaging artifacts but is far enough away to minimizepatient dosage. To determine target beam position Z_(t), ratios ofdetector samples for successive collimator 52 step positions areutilized to determine a detector differential error. A reconstructionerror sensitivity function w(i) then is applied to weight the detectordifferential error. Reconstruction error sensitivity function w(i) isrelated to the percent positive contribution of a detector element 20 asa function of its radial distance from isocenter 24. Function w(i), inone embodiment, is computed from nominal system geometry. In anotherembodiment, w(i) is empirically determined. For example, the followingequations describe an empirical determination of w(i):

b(i)=0.018, 0≦i≦5

b(i)=0.035+0.00075x(i−5), 5≦i≦213

b(i)=0.414+0.00365x(i−213), 214≦i≦n

where i represents detector element position from isocenter 24 and b(i)represents an artifact threshold, i.e. a percent differential error, fora double detector element 20 error. Reconstruction error sensitivityfunction w(i) then is determined in accordance with:

w(i)=0.18/b(i).

A collimator 52 step position SP is determined for which the weighteddetector differential error exceeds a limit L empirically known toproduce image artifacts, for example, 0.04 percent. Target beam positionZ_(t) then is set for the isocenter detector element at a distance justpreceding SP by an amount exceeding applicable tracking loop positioningerror.

Beam position transfer function Z(R) then is determined 110 for a ratioR of an average of outer row 62 to inner row 64 signals for a set ofdetector elements at an extreme end of x-ray fan beam 16. Beam 16positions, determined 106 for each collimator 52 step position, arefitted to the ratio for each collimator 52 step position with afourth-degree polynomial, for example, in accordance with:

Z(R)=a+bR+cR²+dR³+eR⁴

over a suitable ratio range between a maximum and minimum for thesequence of steps.

A valid position measurement range for Z(R) is determined 112 as betweenend limits of the set of collimator 52 step positions for which an errorbetween a beam 16 position determined by Z and an actual beam 16position is less than a predetermined limit, for example, 0.2millimeters. In other embodiments, the predetermined limit is between0.1 millimeters to 0.6 millimeters. In still other embodiments, thepredetermined limit is set at a value between a lower limit just above avalue at which a range of beam 16 position that can be preciselymeasured is too limited, and just below a lower limit that is deemed tocreate tracking errors so large as to unacceptably compromise thebenefits of tracking.

The above described tracking loop senses the signal ratio betweendetector rows and moves system collimation to maintain the x-ray beamvery close to the imaging system detector array edge during patientscanning. As a result, patient x-ray dosage is reduced 20 to 40 percentwithout sacrificing image quality.

Other functions can be utilized in place of beam position transferfunction Z(R) and also in place of reconstruction error sensitivityfunction w(i). In some embodiments, the methods described herein areimplemented by software, firmware, or by a combination thereofcontrolling either computer 36, image reconstructor 34, or both. Also,additional z-detector rows can be provided. In such an embodiment,various combinations of z-detector row signals can be used as the innerand outer row signals, thereby becoming identified as such, or adifferent and/or more elaborate transfer function can be used todetermine a beam position.

It should be understood that system 10 is described herein by way ofexample only, and the invention can be practiced in connection withother types of imaging systems. While the invention has been describedin terms of various specific embodiments, those skilled in the art willrecognize that the invention can be practiced with modification withinthe spirit and scope of the claims.

What is claimed is:
 1. A method for positioning an x-ray beam on amulti-slice detector array of an imaging system, the detector arrayhaving rows of detector elements and configured to detect x-rays inslices along a z-axis, said method comprising the steps of: comparingdata signals representative of x-ray intensity received from differentrows of detector elements; and positioning the x-ray beam in accordancewith a result of the comparison.
 2. A method in accordance with claim 1,wherein comparing data signals representative of x-ray intensityreceived from different rows of detector elements comprises the step ofdetermining a ratio of a sum of signals representative of x-rayintensity received from different rows of detector elements over a timeinterval, and wherein positioning the x-ray beam in accordance with aresult of the comparison comprises the step of positioning the x-raybeam in accordance with the determined ratio.
 3. A method in accordancewith claim 2 wherein determining a ratio of a sum of signalsrepresentative of x-ray intensity comprises the steps of determining asum of signals received from an inner row of the detector array over atime interval, and determining a sum of signals received from an outerrow of the detector array over the time interval.
 4. A method inaccordance with claim 3 wherein the x-ray beam is a fan beam, andpositioning the x-ray beam in accordance with the determined ratiocomprises the step of applying a ratio correction factor to thedetermined ratio to obtain a corrected ratio, and selecting an x-raybeam position as a function of the corrected ratio.
 5. A method inaccordance with claim 4 further comprising the step of selecting theratio correction factor to adjust a relative gain difference between theouter row and the inner row.
 6. A method in accordance with claim 4wherein selecting an x-ray beam position as a function of the correctedratio comprises applying a beam position transfer function to thecorrected ratio to select the x-ray beam position.
 7. A method inaccordance with claim 6 wherein said imaging system includes acollimator configured to collimate and position the x-ray beam, thecollimator having at least one cam having a taper, and said methodfurther comprise the step of determining a focal spot position f inaccordance with$f = {\frac{\left( {Z - C - T_{z}} \right)}{{fm}_{zz}\left( l_{fs} \right)} + C + T_{z}}$

where Z is a position of the beam, C is a current collimator position,T_(z) is a current cam taper, fm_(zz) is a focal spot magnificationfactor and l_(fs) is a focal spot length.
 8. A method in accordance withclaim 6 further comprising the step of determining a new collimatorposition C_(n) in accordance with:$C_{n} = {\frac{\left( {Z_{t} - f} \right)}{{cm}_{i}\left( l_{fs} \right)} + f}$

where Z_(t) is a target beam position at which to maintain the x-raybeam, f is the focal spot position, cm_(i) is a collimator magnificationfactor, and l_(fs) is a focal spot length.
 9. A method in accordancewith claim 8 further comprising the step of positioning the outer row ofthe detector array substantially within a penumbra of the x-ray beam andthe inner row of the detector array substantially within an umbra of thex-ray beam.
 10. A method in accordance with claim 2 wherein the imagingsystem includes a focal spot and said method further comprises the stepof determining an initial focal spot position from a previouslydetermined focal spot position.
 11. A method in accordance with claim 10wherein the step of determining an initial focal spot position from apreviously determined focal spot position comprises the step ofapproximating a thermal focal spot position change as a linear functionof time.
 12. A method in accordance with claim 11 wherein the step ofapproximating a thermal focal spot position change comprisesapproximating the thermal focal spot position change as a97-nanometer-per-second linear function.
 13. A method in accordance withclaim 12 wherein the imaging system includes an x-ray source and furthercomprising the step of clamping the linear function in accordance withan approximation of thermal focal spot position change after a partialcooling of the x-ray source.
 14. A method in accordance with claim 13wherein clamping the linear function comprises the step of clamping thelinear function at approximately twenty percent of focal spot positionchange after complete cooling of the x-ray source.
 15. A method inaccordance with claim 2 wherein the time interval is between 5milliseconds and 50 milliseconds.
 16. A method in accordance with claim2 wherein the time interval is 20 milliseconds.
 17. A method wherein thesteps of claim 2 are performed a plurality of times and the differentrows of detector elements are z-position detectors, and furthercomprising the step of adjusting the time interval downward following az-position detector blockage.
 18. A method in accordance with claim 17wherein adjusting the time interval comprises adjusting the timeinterval from 20 milliseconds to 5 milliseconds.
 19. A method whereinthe steps of claim 2 are performed a plurality of times using an imagingsystem having an x-ray source energized by a signal proportional to agenerating current, and further comprising the step of adjusting thetime interval downward after the x-ray source has been initiallyenergized.
 20. A method in accordance with claim 19 wherein the timeinterval is adjusted from 20 milliseconds to 5 milliseconds after thex-ray source is energized.
 21. A method in accordance with claim 2wherein said step of comparing data signals representative of x-rayintensity received from different rows of detector elements is performeda plurality of times, the different rows of detector elements arez-position detectors, and further comprising the steps of determiningexistence of a z-position detector blockage and suspending performanceof said step of positioning the x-ray beam in accordance with theresults of the comparison after said determination.
 22. A method inaccordance with claim 21 wherein determining existence of a z-positiondetector blockage comprises determining when a signal received from alast data detector element adjacent to the z-position detector is lessthan a value between 0.5 and 0.95 times an expected signal.
 23. A methodin accordance with claim 21 wherein determining existence of az-position detector blockage comprises determining when a signalreceived from a last data detector element adjacent to the z-positiondetector is less than 0.90 times an expected signal.
 24. A method inaccordance with claim 1 wherein said step of comparing data signalsrepresentative of x-ray intensity received from different rows ofdetector elements is performed a plurality of times, and furthercomprising the steps of determining existence of signal corruption, andsuspending performance of said step of positioning the x-ray beam inaccordance with the results of the comparison during existence of signalcorruption.
 25. A method in accordance with claim 24 wherein the imagingsystem includes a collimator and determining the existence of signalcorruption comprises the steps of: determining an actual focal spotlength from a beam position and a collimator position; and comparing theactual focal spot length to a nominal focal spot length.
 26. A method inaccordance with claim 25 wherein said step of positioning the x-ray beamis suspended when a difference of more than an amount between 0.05millimeter and 0.6 millimeter is detected between the actual focal spotlength and the nominal focal spot length.
 27. A method in accordancewith claim 25 wherein said step of positioning the x-ray beam issuspended when a difference of more than 0.1 millimeter is detectedbetween the actual focal spot length and the nominal focal spot length.28. A method in accordance with claim 25 wherein said imaging systemcomprises a rotating gantry on which the detector and an x-ray sourcegenerating the x-ray beam is mounted, said method further comprising thesteps of rotating the gantry and energizing the x-ray source to generatethe x-ray beam, and further comprising the steps of suspending saidrotation of the gantry and said generation of the x-ray beam when adifference of more than an amount between 0.05 millimeter and 0.6millimeter is detected between the actual focal spot length and thenominal focal spot length over an angle between 45° and 360° of gantryrotation.
 29. A method in accordance with claim 25 wherein said imagingsystem comprises a rotating gantry on which the detector and an x-raysource generating the x-ray beam is mounted, said method furthercomprising the steps of rotating the gantry and energizing the x-raysource to generate the x-ray beam, and further comprising the steps ofsuspending said rotation of the gantry and said generation of the x-raybeam when a difference of more than 0.1 millimeter is detected betweenthe actual focal spot length and the nominal focal spot length over 90°of gantry rotation.
 30. An imaging system comprising an x-ray source,multi-slice detector array having rows of detector elements configuredto detect an x-ray beam from said x-ray source in slices along a z-axis,said system configured to: compare data signals representative of x-rayintensity received from different rows of detector elements; andposition the x-ray beam in accordance with a result of the comparison.31. A system in accordance with claim 30, wherein said system beingconfigured to compare data signals representative of x-ray intensityreceived from different rows of detector elements comprises said systembeing configured to determine a ratio of a sum of signals representativeof x-ray intensity received from different rows of detector elementsover a time interval; and wherein said system being configured toposition the x-ray beam in accordance with a result of the comparisoncomprises said system being configured to position the x-ray beam inaccordance with the determined ratio.
 32. A system in accordance withclaim 31 wherein said system being configured to determine a ratio of asum of signals representative of x-ray intensity comprises said systembeing configured to determine a sum of signals received from an innerrow of said detector array over a time interval, and to determine a sumof signals received from an outer row of said detector array over saidtime interval.
 33. A system in accordance with claim 32 wherein saidx-ray beam is a fan beam, and said system being configured to positionsaid x-ray beam in accordance with said determined ratio comprises saidsystem being configured to apply a ratio correction factor to saiddetermined ratio to obtain a corrected ratio, and to select an x-raybeam position as a function of said corrected ratio.
 34. A system inaccordance with claim 33 and further configured to select said ratiocorrection factor to adjust a relative gain difference between saidouter row and said inner row of said detector array.
 35. A system inaccordance with claim 33 wherein said system being configured to selectan x-ray beam position as a function of said corrected ratio comprisessaid system being configured to apply a beam position transfer functionto said corrected ratio to select said x-ray beam position.
 36. A systemin accordance with claim 35 further comprising a collimator configuredto collimate and position said x-ray beam, said collimator having atleast one cam having a taper, and said system is further configured todetermine a focal spot position f in accordance with$f = {\frac{\left( {Z - C - T_{z}} \right)}{{fm}_{zz}\left( l_{fs} \right)} + C + T_{z}}$

where Z is a position of the beam, C is a current collimator position,T_(z) is a current cam taper, fm_(zz) is a focal spot magnificationfactor and l_(fs) is a focal spot length.
 37. A system in accordancewith claim 35 further configured to determine a new collimator positionC_(n) in accordance with:$C_{n} = {\frac{\left( {Z_{t} - f} \right)}{{cm}_{i}\left( l_{fs} \right)} + f}$

where Z_(t) is a target beam position at which to maintain said x-raybeam, f is the focal spot position, cm_(i) is a collimator magnificationfactor, and l_(fs) is a focal spot length.
 38. A system in accordancewith claim 37 further configured to position said outer row of saiddetector array substantially within a penumbra of said x-ray beam andsaid inner row of said detector array substantially within an umbra ofsaid x-ray beam.
 39. A system in accordance with claim 31 wherein saidsystem includes a focal spot, said system being further configured todetermine an initial focal spot position from a previously determinedfocal spot position.
 40. A system in accordance with claim 39 whereinsaid system being configured to determine an initial focal spot positionfrom a previously determined focal spot position comprises said systembeing configured to approximate a thermal focal spot position change asa linear function of time.
 41. A system in accordance with claim 40wherein said system is configured to approximate said thermal focal spotposition change as a 97-nanometer-per-second linear function.
 42. Asystem in accordance with claim 41 further comprising an x-ray source,said system further configured to clamp said linear function inaccordance with an approximation of thermal focal spot position changeafter a partial cooling of said x-ray source cooling.
 43. A system inaccordance with claim 42 wherein said system is configured to clamp saidlinear function at approximately twenty percent of a focal spot positionchange after complete cooling of said x-ray source.
 44. A system inaccordance with claim 31 wherein said time interval is between 5milliseconds and 50 milliseconds.
 45. A system in accordance with claim31 wherein said time interval is 20 milliseconds.
 46. A system inaccordance with claim 31 configured to repeatedly determine said ratioof said sum of signals and to position said x-ray beam, and furtherwherein said different rows of detector elements are z-positiondetectors, and said system is further configured to adjust said timeinterval downward following a z-position detector blockage.
 47. A systemin accordance with claim 46 configured to adjust said time interval from20 milliseconds to 5 milliseconds following said z-position detectorblockage.
 48. A system in accordance with claim 31 configured torepeatedly compare data signals representative of x-ray intensityreceived from different rows of detector elements, further comprising anx-ray source energized by a signal proportional to a generating current,and further configured to adjust said time interval after said x-raysource has been initially energized.
 49. A system in accordance withclaim 48 configured to adjust said time interval from approximately 20milliseconds to approximately 5 milliseconds after said x-ray source isenergized.
 50. A system in accordance with claim 31 configured torepeatedly compare data signals representative of x-ray intensityreceived from different rows of detector elements, said system furtherbeing configured determine existence of a z-position detector blockageand to suspend said x-ray beam positioning upon determining existence ofa z-position detector blockage.
 51. A system in accordance with claim 50wherein said system being configured to determine existence of az-position detector blockage comprises said system being configured todetermine when a signal received from a last data detector elementadjacent to said z-position detector is less than a value between 0.5and 0.95 times an expected signal.
 52. A system in accordance with claim50 wherein said system being configured to determine existence of az-position detector blockage comprises said system being configured todetermine when a signal received from a last data detector elementadjacent to said z-position detector is less than a value 0.90 times anexpected signal.
 53. A system in accordance with claim 31 configured torepeatedly compare data signals representative of x-ray intensityreceived from different rows of detector elements, and furtherconfigured to determine existence of signal corruption and to suspendsaid x-ray beam positioning during existence of signal corruption.
 54. Asystem in accordance with claim 53 further comprising a collimatorconfigured to collimate and position said x-ray beam, and said systemconfigured to determine the existence of signal corruption comprisessaid system being configured to: determine an actual focal spot lengthfrom a position of said x-ray beam and a position of said collimator;and compare the actual focal spot length to a nominal focal spot length.55. A system in accordance with claim 54 configured to suspend saidx-ray beam positioning when a difference of more than a value between0.05 millimeter and 0.6 millimeter is detected between the actual focalspot length and said nominal focal spot length.
 56. A system inaccordance with claim 54 configured to suspend said x-ray beampositioning when a difference of more than 0.10 is detected between theactual focal spot length and said nominal focal spot length.
 57. Asystem in accordance with claim 54 further comprising a rotating gantryon which the detector and an x-ray source generating the x-ray beam aremounted, said system being configured to rotate said gantry and toenergize said x-ray source to generate said x-ray beam, and to suspendsaid rotation of said gantry and generation of said x-ray beam when adifference of more than an amount between 0.05 and 0.6 millimeter isdetected between said actual focal spot length and said nominal focalspot length over an angle between 45° and 360° of gantry rotation.
 58. Asystem in accordance with claim 54 further comprising a rotating gantryon which the detector and an x-ray source generating the x-ray beam aremounted, said system being configured to rotate said gantry and toenergize said x-ray source to generate said x-ray beam, and to suspendsaid rotation of said gantry and generation of said x-ray beam when adifference of more than 0.1 millimeter is detected between said actualfocal spot length and said nominal focal spot length over 90° of gantryrotation.